Shanghai is cracking down on its most tenacious fashion trend. In preparation for the 2010 World Expo, the municipal government has launched a campaign to eradicate running red lights and wearing pajamas in public.

For those of you who have not been wowed by the most cosmopolitan of Chinese cities, Shanghai is known for, among other things, its middle-aged women who saunter onto the street in their sleepwear. Some even venture as far as the subway or the shopping mall.

As the slogan, "Do not go outdoors in pajamas, be a civilized person for the Expo," demonstrates, this tradition does not dovetail with the ideal projection of an international metropolis.

Trouble is, not every Shanghainese believes the practice is uncivilized. There has been sporadic pressure urging locals to abandon the habit, but it refuses to die. Only this time the drum is beating faster because, as one granny put it, "We're so close to the Expo site foreigners may drop by and we've got to put on our best."

So, it's not really about whether we like it, but rather about whether we are liked. Again, it's the quintessential concept of "face" and "saving face".

Not many Chinese are shocked to see a street full of pajama-wearing pedestrians, but if international visitors feel squeamish about it we should stop doing it. Or so the implied rationale for the crackdown goes.

I used to feel wearing pajamas on the streets was inappropriate, but when I dig deeper I can trace it to what an American friend told me: To wear pajamas in public is like going naked. Before that, I didn't even notice it. Pajamas were not something I saw often when I was growing up (where I come from, we sleep in our underwear).

I asked an Australian colleague about this and he told me the sight of someone wearing pajamas on the street would remind him of someone escaping from an asylum.

The pajamas custom, however, is fundamentally different from such bad habits as spitting or running red lights. It does not have a detrimental effect on others.

Before I delve into a defense of pajamas, I'll offer the most common explanations for this Shanghai phenomenon - occasionally seen in other Chinese cities as well but never so ubiquitously.

The number one reason is comfort. Most Shanghainese do not see pajamas as sleepwear. Rather it's casual wear one can slip into after work. Since it does not expose much flesh, people do not feel self-conscious - as they might when wearing, say, bikinis or evening gowns.

Old-style Shanghai residences are often quite cramped. There is a thin line between public and private space and gradually housewives looking for more space subconsciously came to view neighborhood streets as part of their front yard, somewhere they felt comfortable enough to parade around in their pajamas.

Back in the 1930s, when pajamas first came into vogue, they were a sign of affluence and had a certain cachet. Many decades later, this sense of pajamas as a fashion statement lingered. This theory is, to an extent, corroborated by Shanghai-themed movies in which women in pajamas are portrayed as glamorous.

Another popular piece of conjecture runs thus: Pajama wearers do not venture far from home and since they are mostly downtown dwellers they are actually sending a subliminal message about their social status. As you may have heard, Shanghai people are extremely status conscious and the location of their home is an important element of this. Downtown is seen as desirable. In other words, you don't catch a suburban (read, lower-status) person in nightgowns on Nanjing Road, the city's equivalent of New York's Fifth Avenue. So, wearing pajamas is tantamount to pinning a badge declaring: "I'm a classy and authentic Shanghainese."

Each of these hypotheses may have some truth to them. I tend to go for the idea of pajamas as a fashion statement. Shanghai residents would never forfeit class for comfort. Back in the era when most lived in slum-like conditions, they would make sure everything was immaculate the moment they stepped outdoors. They would rather be caught dead than wearing something to be laughed at.

My guess is, when pajamas first came to Shanghai, they made a strong impact and were seen as modish, but their correct use was not widely known. Then came the long period of isolation when such information was hard to come by and eventually pajamas on the street were accepted in an entirely different context, compared with the West.

What the Shanghai government and critics of pajamas on the streets are intent on doing is hitching a ride on the global locomotive and closing the gap between self-perception and outside perception. Something a fashionista, like Anna Wintour, could do with a wave of her hand is now a headache for the administration. It makes me laugh to think of her issuing the edict: "Let them wear Prada!"

Actually, the Shanghai government has adopted a "civilized" approach to the "problem", different to the late 1970s when fashion police armed with scissors forcibly cut the long hair or bell-bottom trousers of passersby. This time, kids have been deployed to shame pajama wearers into submission, with lines like these: "Auntie, your dress looks great, but don't you think wearing something else would make you look better, not to say make our city so much more palatable to international taste?"

Fashion is part of culture. Right now it is Western-centric. What is fitting in one culture may be outrageous in another. An elderly African-American, a neighbor of mine when I was in California, would put on a suit and a tie to go to the front porch to pick up the morning paper. In his mind, anything less would be inappropriate.

To a Chinese old-timer, Western-style evening gowns and the dresses worn for Latin dancing induce embarrassment. But they've come to accept it and other perceived breaches of modesty because they have become used to it through ads, watching TV and so on.

If you shift your position and view it as a cultural quirk, you'll be more amused than disturbed. Maybe pajamas on the streets do not fit with your notion of propriety, but you don't really want China's fashions to replicate, say, Paris, do you? You expect something different, and the difference is culture.

Filmmaker Jiang Wen once told me how shocked he was when, as a child, he stumbled upon a village where grownup women bared their breasts in public. They did not feel awkward, he did. Now the scene is part of a fond memory, devoid of any sexual or negative innuendo.

Speaking of naked bodies, Beijing tried to wipe out the sight of "shirtless guys" for the Olympics. Again, wandering around bare-chested, or with the shirt half-rolled up has its sociological roots, what with the lack of air-conditioning and so on. But it does not really debase the culture.

Like it or not, such a custom is a natural state of being for certain social strata of China. I feel there is a rugged charm to it. Given time, these traditions will evolve. But we should not be tempted to hasten their evolution - or demise. We may wax nostalgic when they are gone.

As for the Shanghai shenanigan, I suggest a different approach: Let the government give pajamas to rural residents in poverty-stricken areas. Television images will instantly put off Shanghainese and they will give up their favorite fashion choice without any prodding.